Influence of local mineral raw materials on the commercial success of Aveiro production of ancient ceramic sugar jars

Ceramic sugar jars played a paramount role in the sugar production cycle, being used for the maturation of the sugar. These pieces were conic having a hole at the bottom, being used specifically for the stage of the purge of the sugar cake. From the 15th until the beginning of the nineteenth centuries, the old pottery centres from Aveiro and Lisbon regions (Portugal) produced heavily these “formas de açúcar” (“sugar jars”), which were exported to sugar production areas, at places as diverse as Madeira, Canaries, Cape Verde, Cuba and Brazil. Mineralogical and chemical data obtained in samples from Aveiro point to local production, using the upper Cretaceous (Maastrichtian) marly (dolomitic) clays and clayey sands as primary raw materials. Ceramics from Barreiro (Lisbon) are generally more silicated and less carbonated, a composition close to the Tagus Cenozoic Basin clays. The higher iron content of Aveiro clays favours the glazing of ceramic paste at lower temperatures, giving better mechanical resistance, which can justify “their best quality”. This study highligths the influence of local mineral raw materials on the technological characteristics of produced sugarware and the actual reasons that allowed Aveiro sugarware production to be competitive to Lisbon (Barreiro) one.


Introduction
Recognition of the source raw materials in ceramics involves a complex array of provenance studies. Mineralogical and geochemical techniques have proved their soundness in investigating the provenance and technology of archaeological ceramics (e.g., Prudêncio and Dias 2014;Rocha and Morgado 2014;Bonis et al. 2018). Local geology is crucial to identifying potential raw materials for ceramic manufacturing. A combination of several analytical techniques can contribute to the identification and characterization of a ceramic raw material. Provenance studies that include mineralogical and/or geochemical fingerprinting are common in applied clay geology and sedimentary geology and can also be applied to reach this research goal.
"Sugar Jars" ceramics are recognized for their peculiar typology ( Fig. 1), having a conical shape, walls of great thickness, absence of flat base and presence of an orifice at the apex.
They were essential parts in the sugar production process (Fig. 2), which justified its large-scale use in the various producing areas (Barros et al. 2006;Sousa 2006;Lourenço and Bugalhão 2006;Morgado et al. 2012). This ceramic element was characterized when sugar had great value in world trade. Portugal dominated a good share of this market thanks to its former colonies (Madeira, Cape Verde, and, above all, NE Brazil), bringing great economic wealth to the country.
During colonial period (from 1500 to 1821), sugar jars production was only authorized in mainland Portugal, being forbidden in the colonies to allow better fiscal control by colonial authorities. Sugar jars production in mainland Portugal ceased with Brazil independence (1821) and the end of the colonial trade system. Only two producing sites (of these ceramics) are known on mainland Portugal (Fig. 3): Barreiro (Barros et al. 2006) and Aveiro (Morgado et al. 2012).
Recent investigations on archaeological occurrences of Mata da Machada (Barreiro), including ceramic ovens, and numerous discoveries in the Aveiro urban area, including entire pieces used as construction materials on building walls (Fig. 4), have provided a rich and varied variety set of samples available for laboratory studies.
Barreiro production was ideally included in the pendular trade between Lisbon and the colonies. However, the export of Aveiro sugarware production, then just a small city of reduced consumption, meant a purposeful coming of practically empty ships (which is evidenced by the existence of ballast stone from those ships in historical buildings of the city, rich in clays but poor in stone for construction). Thus, we presume that Aveiro sugarware production have distinctive technical characteristics undoubtedly related to local raw materials that justified this purposeful coming. Moreover, Aveiro has been a ceramic centre of national paramount importance since Roman times, for both structural red ceramics (bricks and tiles) and finest white ceramics (including porcelain), taking advantage of the local and regional clayey raw deposits (Coroado et al. 1998;Galhano et al. 1999;Marques et al. 2011).
This work aims to study the influence of mineral raw materials (from both manufacturing sites) on the technical characteristics of produced sugarware and the technological development of production processes. For this purpose, sedimentological, mineralogical and geochemical studies were performed on samples from outcrops of the geological formations from which the raw materials were exploited (Trindade et al. 2010a(Trindade et al. , b, 2013Daoudi et al. 2018;Xanthopoulos et al. 2020). That was followed by a study of ceramics found on the two production sites (Aveiro and Barreiro), first of all seeking to identify technological changes in their production and the second to confirm the origin and eventual differences in use (Prudêncio and Dias 2014;Rocha and Morgado 2014;Christidis et al. 2014;Montana 2020;Hein and Kilikoglou 2020;Cordell et al. 2017;Weiner et al. 2020).
Archeometry studies have been dominated by dating, technological characterization and provenance tests, mainly inorganic materials such as stone, ceramics and metals (Prudêncio and Dias 2014;Rocha and Morgado 2014). Studies of the origin of archaeological ceramics through analysis of their elemental composition are based on the assumption that raw materials can be distinguished through an appropriate methodological approach integrating mineralogy and geochemistry (Christidis et al. 2014;Hein and Kilikoglou 2020;Montana 2020). Therefore, it is essential to identify, characterize and distinguish potential raw materials, transformed and altered/degraded, and test the reconstruction of their history of selection, manufacture, uses and changes after application (Cordell et al. 2017). The ceramic process is essentially thermal, producing phase changes, so it can be approached as inducing polymorphism, diagenesis and metamorphism (Weiner et al. 2020). These mineralogical transformations have a potential temperature marker value and comparative analyses between ceramic product and possible raw material have a "reverse engineering" character (Xanthopoulos et al. 2020).
In the case of ceramics of the pottery centre of Aveiro, preliminary studies on historical-documentary analyses were made, followed by estimation of dating by stratigraphic analysis, macroscopic analysis with morphological and typological study, estimation of cooking atmosphere, chromatographic study and textural analysis (Morgado 2009;Morgado et al. 2012;Nobre 2017). Some mineralogical analyses by XRD were also done, allowing the first estimation of firing temperature; chemical analysis was performed by X-ray fluorescence spectrometry (FRX) as well as some simple dynamic compression tests to assess the mechanical resistance (Morgado et al. 2012;Nobre 2017;Moutinho et al. 2019).

Materials
The studied ceramics were found at a subaquatic archaeological site in a channel of Aveiro coastal lagoon (Fig. 5). That allows obtaining some integrally preserved forms. Also, eight sites corresponding to old buildings located in Aveiro urban areas were used as construction materials, usually as fragments, but with some walls constructed with fully integral forms added with traditional one's mortars.
In 1980 a Pottery Center was discovered in the middle of Mata da Machada (Barreiro) forest, excavating the first furnaces known in Portugal from XV/XVI centuries. The   (Morgado et al. 2012) ceramic pieces exhumed led to the identification of the parts produced, including building materials (brick and tile) and industrial ceramics (cookie shapes and forms of Sugar Loaf). The furnace's operation period dates between approximately 1450 and 1530, granted by Numismatic pieces. At Santo António da Charneca (Barreiro), ancient pottery was identified in 1997, found during the development of urbanization. Among the pieces collected stands out a significant set of fragments of forms of sugarloaf. From well-dated archaeological stratigraphy, it is possible to state that the furnace's operation period is situated between the end of the XV and the middle of the XVI centuries. In addition, was also collected in the excavation site a coin of Manuel I, king of Portugal (1495/1521).
Twenty Aveiro ceramics samples were selected for this study, five from the sub-aquatic archaeological site and 15 from several urban sites. In addition, 15 ceramic samples from Barreiro 2 sites were also studied. Finally, sampling was also done on Aveiro and Barreiro local outcrops of the regional lithological units known as having been (some still being) exploited for ceramics production.

Geological setting
In the Aveiro region (Fig. 5), there are huge deposits of heavy clays that belong to the geological formation "Argilas de Aveiro", dated of the Upper Cretaceous (Campanian-Maastrichtian according to Teixeira and Zbyszewski 1976) and belonging to the Lusitanian basin's northern sector (Portuguese western margin). A regression caused the deposition of the "Argilas de Aveiro" Formation, which is composed of marly-sandy clays and resulted from intensive and rapid weathering and erosion that affected rejuvenated continental areas in a temperate climate. As a result, this "Argilas de Aveiro" Formation is characterized by high kaolinite and illite content showing, as a rule, highly degraded structure and irregular interstratifications (Rocha and Gomes 1995;Galhano et al. 1999;Marques et al. 2011).
Cenozoic formations, mainly sedimentary series, characterize the geological setting of the Barreiro study area. Pliocene and Pleistocene have a predominantly sandy  Galhano et al. 1999) sequence, while Holocene deposits are characterized by sandy and clayey lenticular beds with lateral and vertical facies variations (Vicencio et al. 2015). The pottery of Santo António da Charneca and Mata da Machada (Fig. 6), both from the fifteenth-sixteenth centuries, are located on the Marco Furado Quaternary geological formation (QMF on geological maps), being considered as the primary source of ceramic raw materials together with local Pliocene layers, as more recently by industrial ceramic plants ( Barros et al. 2006). Both formations are illite-kaolinite clays/clayey sands, the Quaternary richer in illite, whereas the Pliocene is richer in kaolinite.

Methods
Mineralogical analyses of ceramic pieces (total sample) and clay fractions (under 0.063 mm and 0.002 mm) were carried out by X-ray diffraction, using a Panalytical X'Pert-Pro MPD, Kα Cu (λ = 1,5405 Å) radiation on random-oriented powders (total sample and < 0.063 mm) and oriented aggregates (< 0.002 mm). The oriented aggregates were treated with glycerol and exposed to heat-treatment at 500 °C. Mineralogical composition was assessed using (hkl) peaks (on random powder mounts) for non-clay minerals and (00 l) ones (on oriented aggregates) for clay minerals. The identification of the different mineral phases followed the criteria recommended by Brindley and Brown (1980) and the Joint Committee for Powder Diffraction Standards. The mineralogical semi-quantification of the identified minerals was made through peak areas determination of the specific reflections (Brindley and Brown 1980) and was calculated following the "reflective powers method" according to Galhano et al. (1999) and Oliveira et al. (2002).
Determination of chemical composition was assessed by X-Ray fluorescence using a Panalytical Axios PW4400/40 for major and trace elements; Lost on Ignition (LOI) was also determined (heated at 1000 °C for 2 h).

Results and discussion
The studied ceramics and clayey raw materials (Table 1) show a predominance of silicate detrital minerals, such as quartz, feldspars, and phyllosilicates, accompanied by goethite as accessories and hematite, calcite and dolomite. The clay fractions of the raw materials show illite as the main mineral, accompanied by kaolinite, with smectite (and illite-smectite) in smaller amounts.
Compared with Barreiro raw materials, Aveiro Cretaceous clays are richer in phyllosilicates and slightly poorer in quartz; other distinctive features are the relative higher carbonate content (with exclusive presence) dolomite) and slightly higher presence of goethite and hematite.
Concerning clay minerals content, all clayey raw materials are illite rich, but Aveiro Cretaceous clays show the highest content in kaolinite (along with some smectite). In contrast, Barreiro Quaternary sandy clays are those richer in illite, Barreiro Pliocene clayey sands showing an intermediate clay composition.
All Aveiro studied sugarware show a mineralogical composition very similar to traditional bricks produced in Aveiro, from Cretaceous clays: quartz, phyllosilicates (mainly micas), feldspars (mainly K-feldspars), Fe-oxides (main hematite), carbonates (mainly dolomite). However, they show some distinctive features, such as: relative lower content on phyllosilicates (and almost exclusively micas) and a discrete presence of mullite in some samples.
Studied Barreiro ceramics show significant differences towards Aveiro ones, such as: more quartz, less phyllosilicates, less Fe-oxides, no carbonates and no mullite. Table 2 shows the chemical results (major elements) of the studied ceramics and clayey raw materials.
Raw materials show chemical compositions according to their mineralogical compositions and reflect their main  Barros et al. 2006) differences. For example, Aveiro clays, richer in phyllosilicates and less quartz, show naturally less amounts in SiO 2 and slightly higher in Al 2 O 3 and LOI; they also show relative higher contents in MgO (due to dolomite presence) and Fe 2 O 3 (due to hematite and goethite). Also, Barreiro raw materials are richer in SiO 2 and slightly in alkalis.
All Aveiro ceramic samples show a chemical composition close to traditional bricks produced in Aveiro (Coroado et al. 1998), but with some relevant differences, such as: relative higher content in SiO 2 and Al 2 O 3 and a discrete trend to show higher Fe 2 O 3 content. On the other hand, Barreiro ceramics show SiO 2 higher content, Al 2 O 3 and Fe 2 O 3 lower contents and CaO (and MgO) significantly lower contents. Mineralogical composition of Aveiro sugar jars are quite similar to local red ceramics typical raw materials, historically exploited on Upper Cretaceous marly (dolomitic) clays, in which the common clay minerals association is kaolinite plus illite, in some more red layers followed by illite-smectite, and usually also rich in goethite and hematite (Rocha and Gomes 1995).
The persistence of dolomite is coherent with this maximum firing temperature (≈ 800 °C) which is not sufficient to achieve the total decomposition of this mineral that currently occurs in the local clays (Trindade et al. 2010b); dolomite is absent or very discrete in heavy clays of any other Portuguese clay deposit (Rocha and Gomes 1995;Coroado et al. 1998;Marques et al. 2011;Trindade et al. 2013).
On the other hand, the relative lower content on phyllosilicates and the discrete presence of mullite in some samples point to a slightly higher firing temperature (more kaolinite, later production).
Red clays (commonly more kaolinitic) should have been increasingly used in advantage over green clays. Red clays are also richer in Fe-oxides, whereas green clays are richer in dolomite. Relative higher content on Si and Al and the discrete trend to show higher Fe contents, shown by the same samples referred previously, reinforce this consideration (Amaral et al. 2020). It is well known that high iron contents favour the vitrification of the ceramic bodies at lower temperatures, which able the ceramic bodies to acquire good values of mechanical resistance at lower firing temperatures (Amaral et al. 2020;Moutinho et al. 2019). Compressive strength analysis of Aveiro ceramics sugar jars pieces shows values higher (mean 9.5 MPa) than common structural red ceramics (Moutinho et al. 2019). Mineralogical and chemical composition of Lisbon (Barreiro site) sugar jars are similar to local red ceramics typical raw materials, historically exploited on Barreiro (the Portuguese word for clay quarry) Pliocene sandy clays, in which the common clay minerals association is illite + kaolinite + smectite. Furthermore, in the Lisbon region (Barreiro site), sugar jars, the absence of high-temperature phases and the poor contents on Fe oxides explain the lower quality of the firing products.

Conclusions
The obtained mineralogical and chemical results have given important information about the composition of the studied ceramics and their raw materials. Aveiro and Barreiro ceramics' chemical and mineralogical characteristics are similar to surrounding clayey formations, pointing to local production using only local raw materials.
Ceramic pieces from Aveiro are well related with upper Cretaceous (Maastrichtian) marly (dolomitic) clays and clayey sands, whereas ceramics from Barreiro (Lisbon) are in general more silicated and less carbonated, a composition close to Pliocene clayey sands and Quaternary sandy clays.
The presence (even relatively discrete) of high-temperature mineral phases on Aveiro ceramic pieces points to higher firing temperature, explaining the higher quality of the firing products, namely in what concerns compressive strenght. Actually, the higher iron content of Aveiro clays favours the glazing of ceramic paste at lower temperatures, giving them better mechanical resistance, which can justify "their best quality".
This study highligths the influence of local mineral raw materials on the technological characteristics of produced sugarware and the actual reasons that allowed Aveiro sugarware production to be competitive to Lisbon (Barreiro) one.